The most widely used procedure for converting hot rolled stainless steel (hot band) into finished cold rolled product suitable for the marketplace consists of the following steps: (1) annealing; (2) pickling; (3) coil build-up (including welding similar coils end-to-end to make a large coil, welding of leader strips "tails," to both ends of the coil, and trimming the edges of the strip); (4) cold breakdown on a reversing rolling mill; (5) intermediate annealing and removal of "tails"; (6) cold finish rolling on a reversing rolling mill; (7) final annealing; and (8) temper rolling.
Unlike carbon steels, which usually undergo little or no work hardening during hot rolling, stainless steel is typically work hardened when it comes from the hot mill, the hardening corresponding to about 10% to 20% cold reduction.
A given plant may not follow the above-described procedure exactly. For example, some plants do not utilize "tails." Also, strip grinding facilities are needed in many cases to repair surface defects in the hot band.
In general, intermediate annealing following a first stage of cold rolling has been required when the total reduction in thickness from hot band to finished product exceeds approximately 70% when Sendzimir 20-high cluster mills are used for cold rolling (even less if 4-high mills are used). This is because the strip work hardens as it is deformed. In recent years, due to the high cost of energy, great efforts have been made to reduce the requirement for intermediate annealing, despite the fact that average finished thickness has been trending downwards. This has been achieved by: (a) ordering hot band as thin as is required to eliminate the need for the intermediate anneal and (b) by taking greater reductions on the strip than 70% before annealing the material. For the most common 18-8 stainless steel alloy (18% chromium, 8% nickel content) total reductions of 80% are quite common and up to 90% are not unheard of.
The result of this is that the typical stainless steel cold mill now rolls over 80% of its product without an intermediate anneal, as compared with perhaps 20%, ten or fifteen years ago.
There are some disadvantages to this approach. First of all, lighter gauge hot band is more expensive. Secondly, the hot band is subject to a greater percentage variation in thickness along its length due to the temperature difference from end-to-end, (the tail end of the coil spending more time from leaving the furnace to being rolled than the nose end, and the thinner the gauge being rolled, the greater the time difference and resulting temperature difference). It should be noted that in hot rolling, the cooler the strip is, the harder it becomes, and the bigger the deflection of the mill structure.
Thirdly, further disadvantages stem from taking total cold reductions as high as 80% or 90%. These include increased problems with edge cracking, more frequent breaks, and more difficulty in producing good strip flatness, these difficulties resulting from the increased hardness and reduced ductility of the strip at high total reductions.
A further disadvantage arises from the elimination of the intermediate anneal, this being that it is much more difficult to obtain high gauge accuracy. This is due directly to the gauge variation in the hot band. Usually, an AGC (automatic gauge control system) is used on the cold mill in order to help the mill "iron out" the gauge variations. The variation in entry gauge causes variation in the roll separating force. This results in a corresponding variation in deformation of the mill structure, which causes a corresponding variation in roll gap, and hence, exit gauge. For example, if the incoming gauge increases, it forces the work rolls further apart (by an amount inversely proportional to the stiffness of the mill structure) and this increases the roll gap, and hence the exit gauge.
The AGC system is used to sense the variation in exit gauge (or in roll gap, or in elongation) and to adjust the mill screw down in order to keep this variation to a minimum. At first sight, it would appear that if a good AGC system is used to "iron out" the gauge variation on the first pass on the reversing mill, it should not be necessary to use the AGC on later passes, because the entry gauge should be uniform on the second pass. Unfortunately, this is not the case because there will be a variation in strip hardness along the length of the strip rolled during the first pass, corresponding to the initial variation in strip thickness. This is because the initially thicker portions of the strip must undergo additional work as compared to other portions, and as a result become more work hardened.
Therefore, if no AGC is used during the second pass, the variations in hardness of the strip coming to the cold mill will cause corresponding variations in roll separating force, mill deflection, roll gap and thus exit gauge. In short, a cold rolling mill can only eliminate gauge variations or hardness variations. It cannot eliminate both.
For these reasons, the AGC must be used on every pass, and the performance of the AGC is limited by the big variation in entry gauge and/or hardness for which it must compensate on every pass. It should be noted here that gauge variations from end-to-end of a hot rolled stainless steel coil of up to 10% are not unusual, and fairly rapid variations in gauge (caused by "skid marks") of 2% or 3% may also occur at several points in the coil. "Skid marks" are portions of the coil which correspond to the parts of the slab which rested on the skids in the reheat furnace, prior to delivery to the hot mill used to convert the slab to hot band, these parts being cooler than the adjacent parts of the slab when they are rolled. Now, when an intermediate anneal is adopted, it is possible to eliminate the hardness variation along the coil. Therefore, if the AGC is used to give reasonable gauge accuracy on the last Pass before the intermediate anneal, then the strip delivered from the intermediate annealing furnace will have the same reasonable gauge accuracy, but will have virtually no hardness variations. Thus, the AGC has very little work to do on the subsequent passes (the finishing passes) on the reversing mill, so that very high levels of performance can be achieved.
By eliminating the intermediate anneal, this mechanism is lost, and there is a resulting degradation in gauge accuracy in the finished Product. This can result in a large cost penalty, because stainless steels are very expensive materials, and a loss in yield of, say, one-half percent, could result in annual revenue loss of millions of dollars for a typical 50" or 60" mill. Note that, if a minimum gauge is specified, and the gauge tolerance achieved is plus or minus 1%, then the average gauge must be set 1% higher than the minimum. On the other hand, if the tolerance achieved is plus or minus one-half percent, then the average gauge needs only to be set one-half per cent higher than the minimum.
One object of the present invention is to counteract this degradation in gauge accuracy caused when the intermediate anneal is eliminated. A further object is to reduce the incoming gauge of strip delivered to the reversing mill so that, for a given hot band thickness and finish strip thickness, the total reduction applied by the cold rolling process can be reduced, thus reducing the incidence of edge cracks and strip breaks. Alternatively, for a given finished strip thickness, the objective is to enable a hot band of greater thickness (and hence of lower cost, and subject to smaller percentage thickness variation) to be used.